1. Field of the Invention
The present invention relates to a technology for detecting a position of a toner pattern in an image forming apparatus.
2. Description of the Related Art
A variety of image forming apparatuses use toner to form images, i.e., they form toner images. Example of such image forming apparatuses are analog image forming apparatuses, digital image forming apparatuses, black-and-white copiers, color copiers, printers, plotters, facsimile machines, and, multifunction printers (MFPs).
An image that is formed by such image forming apparatuses is called “toner image”. As is widely known, to form a desirable toner image on a recording medium, such as a paper sheet, it is necessary to control a position of the toner image accurately.
For example, in an image forming process of transferring the toner image from a photosensitive element having photoconductive properties onto the recording medium, and fixing the toner image to the recording medium, it is necessary to accurately transfer the toner image from the photosensitive element onto a target position (e.g., center position) on the recording medium.
The accurate transfer cannot be implemented without proper control over the position of the toner image on the photosensitive element with respect to the recording medium to which the toner image is to be transferred.
In a case of forming a multi-color image or a color image by superimposing the toner images in different colors on each other, especially, it is necessary to accurately superimpose, under the control over the position of each of the various-colored toner images, the toner images on each other.
Inaccurate position alignment among the toner images to be superimposed causes formation of an undesirable image due to registration mismatching in which timing to write the image is mismatched, incorrect magnification that causes an incorrect length of the image, color shift in which the relative positions of the toner images are not matched accurately with each other, etc.
To control the position of the toner image properly, a method of detecting a position of a toner image dedicated to a positional detection (hereinafter, “toner pattern”) has been widely used. In the method, the toner pattern is formed first, and then the position of the toner pattern is detected from a change in a detection light reflected from the toner pattern (see, for example, Japanese Patent Application Laid-open No. 2003-84530, Japanese Patent Application Laid-open No. 2004-309292, and Japanese Patent Application Laid-open No. 2002-72612).
The toner pattern is formed under conditions the same as those under which a toner image as a product of the image formation (hereinafter, “product toner image”) is formed. Therefore, it is possible to expect the position of the product toner image based on a result of the detected position of the toner pattern. The image formation conditions can be adjusted based on the result of the detected position of the toner pattern in such a manner that the product toner image is formed on a correct position.
An optical device that emits the detection light to the toner pattern and receives the detection light reflected from the toner pattern is called a reflection-type optical sensor.
Various types of reflection-type optical sensors are known in the art (see Japanese Patent Application Laid-open No. 2003-84530, Japanese Patent Application Laid-open No. 2004-309292, and Japanese Patent Application Laid-open No. 2002-72612).
A typical reflection-type optical sensor includes a light-emitting unit that emits the detection light and a light-receiving unit that receives the reflected light. The light emitting unit includes one or two light-emitting elements. The light-receiving unit includes one or two light-receiving elements (e.g., photodiodes (PDs) or phototransistors).
Light-emitting diodes (LEDs) are typically used as the light-emitting elements. Each of the LEDs emits the detection light of a spot size that is smaller than the toner pattern onto the toner pattern.
The toner pattern is formed, for example, on a transfer belt. The toner pattern moves as the transfer belt rotates. A direction in which the transfer belt moves due to the rotation is called a sub-direction, and a direction perpendicular to the sub-direction is called a main-direction. In a system in which electrostatic latent images are formed through optical scanning, the main-direction corresponds to the main-scanning direction, and the sub-direction corresponds to the sub-scanning direction.
An electrostatic latent image corresponding to the toner pattern is formed on a photosensitive member by optically scanning a surface of the photosensitive member with an electrostatic-latent-image forming unit, and the electrostatic latent image on the surface of the photosensitive member is then developed into the toner pattern. The toner pattern on the photosensitive member is then transferred onto the transfer belt, and is moved in the sub-direction with the rotation of the transfer belt. When the toner pattern enters a detection area, the toner pattern is exposed to a spot of the detection light from the reflection-type optical sensor.
The spot size of the detection light is typically about 2 millimeters (mm) to 3 mm in diameter.
A typical toner pattern for the position detection in the main-direction includes a parallel line pattern that is parallel to the main-direction and a slant line pattern that is inclined to the main-direction. The parallel line pattern and the slant line pattern are aligned in the sub-direction. The position of the toner pattern in the main-direction is detected from a difference between a time when the parallel line pattern is detected and a time when the slant line pattern is detected.
If a part of the spot of the detection light falls out of the line patterns, because of a large positional mismatch between the reflection-type optical sensor and the toner pattern in the main-direction, it is difficult to correctly detect the position of the toner pattern in the main-direction.
Assume, for example, that one light-emitting element emits one spot of the detection light, one light-receiving element receives the reflected light, and the position of the toner pattern (the line patterns) is detected from a difference between a specular reflection light and a diffuse reflection light. It is assumed that the detection light reflected from a region where there is no toner pattern is received as the specular reflection light. The detection light reflected from the toner pattern is received as the diffuse reflection light.
If a part of the spot of the detection light falls out of the line patterns, the part of the spot that falls on the region where there is no toner pattern is specularly reflected, and the specular reflection light is received at the light-receiving unit. On the other hand, the other part of the spot that falls within the line patterns is diffusely reflected.
To detect the line patterns from the intensity of the light received at the light-receiving elements without fail, it is required to suppress a value of the intensity to a threshold or lower by the diffuse reflection by the line patterns. Receiving of the specular reflection light from the region out of the line patterns increases the intensity of the received light, which may disadvantageously increase the intensity higher than the threshold. The specular reflection light can be a factor of a detection-signal error, i.e., the specular reflection light can affect an adverse effect on the detection of the correct position of the toner pattern.
To solve this problem, in the conventional technologies, the toner pattern (the line patterns) is formed to have a length in the main-direction from about 15 mm to about 20 mm long enough to receive the whole spot of the detection light without fail, thereby preventing a part of the spot of the detection light to fall out of the toner pattern.
In the image forming apparatus, especially, the color image forming apparatus, the detection of the position of the toner pattern is performed as a maintenance activity necessary for the image forming apparatus to perform the image forming process properly, and thereby form a high-quality image.
Because the detection of the position of the toner pattern is performed as the maintenance activity separated from a main activity, i.e., an image-forming process, the image formation as the main activity cannot be performed during the detection of the position of the toner pattern.
When the electrostatic latent image to be developed to the toner pattern is written by the optical scanning, time that it takes for the optical scanning is directly proportional to the size of the toner pattern. In other words, the larger the toner pattern is, the lower the operating efficiency of the image formation becomes.
Moreover, because a total amount of the toner is fixed, as an amount of the toner to be used for the toner pattern increases, an amount of the toner to be used for the product toner image decreases, disadvantageously. The larger the toner pattern is, the more the toner is consumed for the toner pattern.
In this manner, the conventional manner of detecting the position of the toner pattern have the two disadvantages, i.e., the low operating efficiency and the large toner-consumption amount for the toner pattern.